I have now removed the PSU/timer board from the chassis so can provide a bit more detail on the crystal. It is a Motorola K1100A 50.00MHz. The nearest datasheet I could find was this although its for a 70MHz K1100.
The output load is specified as 10 TTL. Not sure what that means, but hoping it means up to 10 x TTL gates? It is oblong, has 4 pins spaced 15mm apart lengthwise and 7.5mm apart across with width of the device. It seems only 2 pins are actually used with one being connected to the 4 gates as shown. The other end connects directly to the 5V rail.
I just also realized that I had been focused on U128 and haven't really looked at what U230 is doing.
It seems there are plenty of SMD crystal devices available cheaply, but the metal cased thru-hole versions are a little bit hard to come by. RS and Farnell only seem to stock SMD versions. Not sure about the ones from the far east.
Looks like it might be worth re-capping the 5V section of the PSU at least. Caps are a bit weird with 3 pins. I think that the pin at the closed end is just for stability though.
